Classifications

• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
  • C04B41/45—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements
  • C04B41/50—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements with inorganic materials
  • C04B41/5025—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements with inorganic materials with ceramic materials
  • C04B41/5037—Clay, Kaolin
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
  • C04B41/009—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone characterised by the material treated
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
  • C04B41/45—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements
  • C04B41/50—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements with inorganic materials
  • C04B41/5053—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements with inorganic materials non-oxide ceramics
  • C04B41/5062—Borides, Nitrides or Silicides
  • C04B41/507—Borides
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
  • C04B41/80—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone of only ceramics
  • C04B41/81—Coating or impregnation
  • C04B41/85—Coating or impregnation with inorganic materials
  • C04B41/87—Ceramics
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
  • C04B41/80—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone of only ceramics
  • C04B41/81—Coating or impregnation
  • C04B41/89—Coating or impregnation for obtaining at least two superposed coatings having different compositions
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B11/00—Obtaining noble metals
  • C22B11/06—Chloridising
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B21/00—Obtaining aluminium
  • C22B21/06—Obtaining aluminium refining
  • C22B21/064—Obtaining aluminium refining using inert or reactive gases
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B9/00—General processes of refining or remelting of metals; Apparatus for electroslag or arc remelting of metals
  • C22B9/05—Refining by treating with gases, e.g. gas flushing also refining by means of a material generating gas in situ
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS, OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
  • F27D27/00—Stirring devices for molten material
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B2111/00—Mortars, concrete or artificial stone or mixtures to prepare them, characterised by specific function, property or use
  • C04B2111/00474—Uses not provided for elsewhere in C04B2111/00
  • C04B2111/0087—Uses not provided for elsewhere in C04B2111/00 for metallurgical applications
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS, OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
  • F27D1/00—Casings; Linings; Walls; Roofs
  • F27D1/0003—Linings or walls
  • F27D1/0006—Linings or walls formed from bricks or layers with a particular composition or specific characteristics
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS, OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
  • F27D1/00—Casings; Linings; Walls; Roofs
  • F27D1/16—Making or repairing linings increasing the durability of linings or breaking away linings
  • F27D1/1678—Increasing the durability of linings; Means for protecting
  • F27D1/1684—Increasing the durability of linings; Means for protecting by a special coating applied to the lining
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/10—Reduction of greenhouse gas [GHG] emissions
  • Y02P10/134—Reduction of greenhouse gas [GHG] emissions by avoiding CO2, e.g. using hydrogen
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/20—Recycling
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/25—Process efficiency

Definitions

• the invention relates to an apparatus for treating molten metal, in particular for the purification of molten metal, such as molten aluminium, magnesium, steel, cast iron or copper, having a wear-exposed component, such as a stirrer for dispersing a purifying fluid; and a method for treating molten metal with such an apparatus .
• molten metal such as molten aluminium, magnesium, steel, cast iron or copper
• a wear-exposed component such as a stirrer for dispersing a purifying fluid
• aluminium as well as other metals such as magnesium, steel, cast iron or copper need to be of such high purity that the produced metal needs to undergo a purification process.
• Such purification process usually takes place immediately after the production of the molten metal or during a recycling process .
• Hall-Heroult cells is typically between 99.7 to 99.9%.
• the impurities present in the produced aluminium are mainly silicon, zinc, magnesium, manganese and titanium, traces of copper chromium, gallium, sodium, lithium, calcium, vanadium and boron, as well as oxides, carbides and hydrogen (D.G.Altenpohl, "Aluminium: Technology, Applications, and Environment", 1998, 6th Edition, The Aluminum Association Inc., Washington D.C., p. 15).
• One process for purifying molten metal, in particular molten aluminium includes imparting a rotation to the molten metal around a substantially vertical axis to separate the impurities by centrifugal or gravitational force.
• US Patents 4,760,066 (Eidem) and 5,106,411 disclose such a process in which the rotation is produced by a rotational magnetic field applied to the molten metal.
• WO99/60177 Terojer
• the vessel containing the molten metal is rotated, this process being used to purify the molten metal or to separate it from a second molten metal.
• Another process for purifying molten metal, in particular aluminium consists of passing a purifying additive which may be a halide gas such as chlorine or fluorine, or nitrogen or argon. These gases remove hydrogen, sodium, lithium, calcium and magnesium from the molten metal.
• a purifying additive which may be a halide gas such as chlorine or fluorine, or nitrogen or argon.
• US Patent 3,887,172 discloses an apparatus for the purification of molten steel or cast iron by injecting therein argon and/or nitrogen to reduce the level of silicon and for desulfurisation, dephosphorisation and de- oxidation.
• purifying gas bubbles are finely and homogeneously dispersed in the molten metal for 200 to 300 seconds or longer.
• Gas is finely divided into small bubbles in the molten metal with suitable stirrers, such as rotating dispersers as disclosed in US Patent 3,743,263 and 3,870,511 (both in the name of Szekely) .
• rotors such as "pump action rotors” may be provided with internal cavities communicating with the molten metal bath and with a purifying gas under pressure, the purifying gas being mixed with the molten metal inside the rotor and evacuated therefrom into the molten metal bath, as described for example in US Patents 4,634,105 (Withers/Pattle) , 5,527,381 (Waite/Dumont) and 5,660,614 (Waite/Lavoie/Dube/ Dumont) .
• the stirrers or rotating dispersers immersed in the molten metal and rotating therein are usually made of carbon, in particular graphite.
• stirrers or dispersers are eroded by friction with the molten metal in which they are rotating and rapidly oxidise above the meltline of the molten metal at temperatures exceeding 450°C.
• conventional stirrers or dispersers typically neck down at the region of the meltline.
• the stirrers are in addition exposed to corrosion and/or oxidation when extracted from the molten metal after purification thereof and before immersion into a new molten metal to be purified.
• Conventional stirrers or dispersers used in batch processes quickly cone down along their length. Therefore, such rotating stirrers or dispersers need to be frequently replaced.
• stirrer or disperser parts consists of covering them with a refractory sleeve, e.g. made of alumina, resistant to oxidation and wear.
• a refractory sleeve e.g. made of alumina
• Such sleeves are rigid and can only protect a limited number of shapes corresponding to the internal shapes of the sleeves.
• a conventional protection method consists of impregnating the stirrers or dispersers with aluminium orthophosphate under pressure in autoclaves and limiting their exposure to oxygen during operation by flushing an inert gas, e.g. argon, in the process chamber.
• an inert gas e.g. argon
• aluminium orthophosphate impregnation may double the lifetime of the stirrers or dispersers, as demonstrated in the comparative examples below, such lifetime is still very short and the stirrers or dispersers are still quickly consumed. Furthermore, such an impregnation with aluminium orthophosphate does not efficiently protect stirrers or dispersers against erosion caused by contact with the molten metal which is in relative motion with the stirrers or dispersers . Therefore, there is still a need to enhance protection of the stirrers, dispersers and other components which come into contact with the molten metal against erosion, oxidation and corrosion, and reduce the costs caused by the consumption of stirrers or dispersers during operation by increasing their lifetime.
• a general object of the invention is to provide coatings on components, in particular carbon or carbon-based components, used in metallurgical processes, which components during operation are in moving contact with molten metal, the coatings being such as to protect the components against erosion, oxidation and corrosion.
• An object of the invention is to provide an apparatus for purifying molten metal, in particular molten aluminium, magnesium, steel, cast iron or copper, provided with a component, such as a carbon-based stirrer, having an improved resistance to erosion, oxidation and corrosion.
• Another object of the invention is to provide a carbon- based stirrer having an improved resistance to erosion which can be fitted in existing molten metal purifying apparatus.
• a further object of the invention is to provide a carbon-based stirrer of a molten metal purifying apparatus protected against oxidation and corrosion when it is extracted from the molten metal in a batch process .
• Another object of the invention is to provide a carbon- based stirrer for a molten metal purifying apparatus provided with a low cost and easily applied protection.
• Yet another object of the invention is to provide a method for purifying molten metal in a molten metal purifying apparatus operating with a protected component, such as a carbon-based stirrer.
• the apparatus comprises a component exposable to molten metal to be treated and means for imparting a rotary motion to the molten metal about a substantially vertical axis.
• the apparatus is so arranged that during use at least part of a wear-exposed surface of the component is temporarily or permanently in contact with molten metal, the contacting molten metal being in motion relative to the wear- exposed surface.
• the wear-exposed surface is coated with a slurry-applied protective coating of refractory material in a heat stable binder, in particular an inorganic binder, protecting the wear-exposed surface against erosion, oxidation and corrosion.
• heat stable means that the binder does not lose its integrity at the operating temperature at which the molten metal is treated, i.e. at or above the melting point of the metal .
• Such a coating protecting these components provides the following advantages . 1) The coating can be applied to a component of any shape, even complex shapes, without corrupting the original design.
• the coating can be applied to selected parts of the component, the thickness of the coating being adaptable to the wear exposure of the selected part.
• a typical coating thickness is in the range of 100 to 1500 micron, in particular 250 to 800 micron.
• the coating can be applied by simple techniques, e.g. painting, brushing, spraying and dipping, and does not require complex application devices such as autoclaves.
• the overall wear resistance and oxygen impermeability of the coating is not affected at elevated temperatures, even above 450°C or higher, e.g. above the melting point of magnesium, aluminium, copper or iron.
• the coating is sufficiently resistant to and protective against oxidation that no particular inert atmosphere is needed during use, even at high temperature. This feature is particularly advantageous since older systems which are not designed to operate under inert atmosphere may be economically retrofitted.
• the coating is not merely oxidation or corrosion resistant but also resistant to erosion. 7) The coating substantially increases the possible duration of use of the component as shown in the comparative examples below.
• the coating fully protects the component which can be re-coated and re-used after the coating is worn or needs otherwise to be replaced.
• the coating is suitable to protect numerous substrate materials, e.g. carbon-based materials, such as graphite, carbide materials, such as silicon carbide, and metals, such as iron, steel and titanium, the substrates being thereby protected particularly against chemical attack and dissolution.
• substrate materials e.g. carbon-based materials, such as graphite, carbide materials, such as silicon carbide, and metals, such as iron, steel and titanium, the substrates being thereby protected particularly against chemical attack and dissolution.
• the protective effect of the coating is such that different materials may be used, in particular grades of carbon cheaper than dense graphite.
• the coating has such a smooth surface that no unwanted elements carried in and/or on the molten metal get caught on the surface and carried between batches of treated metal. Hence, the coating needs less or cleaning during use.
• the apparatus according to the invention can be used for separating molten metal from impurities and/or separating constituents of an alloy metal by centrifugal and/or gravitational force.
• the coated part of the coated component may be made of carbon-based or carbide-based material, in particular petroleum coke, metallurgical coke, anthracite, graphite, amorphous carbon or mixtures thereof.
• the coated part of the coated component can be made of metal-based material.
• the apparatus may comprise one or more of such coated components.
• Suitable components include vessels for containing the rotating molten metal, stators that in use dip in the molten metal and are arranged to deliver treating fluid into the molten metal, rotatable stirrers arranged to dip in and rotate the molten metal during operation, and other dispersers or components thereof and.
• the apparatus comprises a rotatable stirrer that is surrounded in the molten metal by a stator, e.g. a tube-like stator, the stirrer extending through the stator and protruding therefrom into the molten metal.
• the rotor and the stator define a passage between them for the delivery of the treating fluid.
• the apparatus comprises means for generating a rotational magnetic field in the molten metal, usually arranged around a vessel containing the molten metal.
• Some coated components like stirrers have an upper and a lower part, the lower part being during operation partly exposed to molten metal up to a meltline.
• the component has an interface portion extending from below to above said meltline which may be coated with the slurry-applied protective coating.
• the apparatus is arranged for the purification of molten metal, in particular the removal of impurities from molten metal and/or degassing molten metal.
• the apparatus comprise a vessel for containing molten metal to be purified or is associated during use with such a vessel.
• the apparatus has means for injecting a fluid, in particular a gas, a liquid, and/or a flowable solid, such as particulate e.g.
• the stirrer has an upper part engaged with a rotary drive means and a carbon-based lower part which during operation is partly immersed in molten metal up to a meltline.
• the carbon-based lower part of the stirrer has an interface portion which extends from below to above the meltline, the interface portion being coated with the slurry-applied protective coating.
• the coating offers the advantage of being suitable to protect any stirrer, including prior art stirrers of complex shapes and stirrers disclosed in the background art discussion.
• the protective coating can be applied selectively onto the stirrer and tailored to control the lifetime of the stirrer.
• the coating can protect any kind of stirrer design in any operating conditions, and be adapted to the type and grade of the carbon-based material to which it is applied.
• the entire carbon-based lower part of the stirrer or other coated component, or even substantially the entire stirrer or component, may be fully coated with the refractory material to improve its resistance. However, it has been found sufficient to coat the stirrer or component only on the meltline region where protection is most needed.
• the coating may also be selectively applied to other parts of the stirrer or component, depending on its design and operating conditions.
• the upper part of the stirrer or other coated component may be coated with a thin coating of refractory material against oxidation and corrosion, while the lower part of the stirrer or component is preferably coated with a thick coating of refractory material against erosion, oxidation and corrosion.
• the upper part of the stirrer or component which is engaged with the drive means, and which is not exposed to molten metal may be made of long-lasting materials other than carbon, in particular oxidation and temperature resistant metals, such as suitable kinds of steel, and/or ceramics.
• the stirrer or other coated component may be a two part assembly having a coated carbon-based lower part assembled to a long-lasting upper part.
• the carbon- based lower part of the stirrer needs to be replaced, in particular when it is eroded, replacement may be limited to the carbon-based lower part by substituting the used lower part with a new one, which enables cost savings.
• the protective coating of the stirrer or other coated component may comprise a refractory hard metal selected from borides of titanium, zirconium, vanadium, tantalum, nickel, molybdenum, chromium and iron, in particular titanium diboride.
• the protective coating can also comprise at least one carbide or oxide of aluminium, silicon, titanium, zirconium, vanadium, tantalum, nickel, molybdenum, chromium, copper and iron, or a combination thereof, in particular corundum or fused alumina.
• the protective coating may be applied from a slurry, in particular a colloidal slurry, containing particulate reactant or non-reactant substances, or a mixture of particulate reactant and non-reactant substances, which when the stirrer is heated at a sufficient temperature, usually above 500°C, form the protective coating by reaction sintering and/or sintering without reaction.
• the coating may be consolidated by fusion and solidification of one or more of its constituents .
• the slurry for such a coating may comprise at least one colloid or an inorganic polymer, for example selected from colloidal or polymeric silica, alumina, yttria and ceria.
• the slurry consists of preformed particulate titanium diboride in colloidal alumina and/or colloidal or polymeric silica.
• the colloidal slurries described in US Patents 5,364,513 and 5,651,872 and other non-sintered slurries may be improved by the addition of polyethylene glycol which increases the viscosity of the slurry and enhances its capacity to be applied by painting as well as the adherence and coherence of the final protective coating.
• the liquid content of the colloidal slurry may contain between 0.5 and 6 weight% , typically 2 to 5 weight%, polyethylene glycol (PEG 300) or a corresponding amount of a polyethylene glycol of different concentration.
• the slurry may be applied to the component by painting, spraying or dipping.
• Use of standard mixing tanks for keeping the slurry in continuous motion without introduction of air bubbles is also possible but neither necessary nor preferred.
• the rheology of the slurry is such that substantially no settling takes place and the slurry remains in a state of readiness without using expensive and elaborate mechanical mixing device. It is usually sufficient to simply dip and agitate or rotate the component in the static slurry.
• the layer can be dried in air at room temperature or at higher temperature, typically 200° to 300°C. The drying process involving heating should be controlled so that the coating does not mud crack.
• the coating should be heat treated at a temperature of at least about 450 to 500°C to permit consolidation by sintering the coating on the component before use or in-situ, or by melting and then solidifying at least one constituent of the coating.
• the melting point of the coating should be above the temperature of operation of the component.
• the consolidation temperatures may be in the range of 500° to 1500°C. The temperature of consolidation by sintering tend to be lower than by melting and solidifying.
• the protective coating extending at least over the part of the surface of the component which is inside the oven.
• the coating may be heat treated using a hot collar which is not larger than the coating.
• the coating preferably extends beyond the heating element during the firing step.
• the coating should extend above the meltline to a level at which the temperature is sufficiently low that no substantial oxidation of un-coated parts of the substrate occurs.
• the drying and heat treatment of the coating may be achieved in a single heat exposure before use of the component. However, if the component is to be consolidated in-situ, drying should take place before using the component.
• the upper part of the component, in particular of the stirrer, especially when it is carbon-based, may be protected from an oxidising and/or corrosive atmosphere with a coating and/or impregnation of a phosphate of aluminium, such as monoaluminium phosphate, aluminium phosphate, aluminium polyphosphate, aluminium metaphosphate, aluminium orthophosphate and mixtures thereof .
• the upper part of the component, in particular the stirrer, especially when it is carbon-based, may also be protected against an oxidising or corrosive atmosphere by coating and/or impregnating it with a boron compound, such as a compound selected from boron oxide, boric acid and tetraboric acid.
• a boron compound such as a compound selected from boron oxide, boric acid and tetraboric acid.
• the extent of the slurry-applied protective coating above the meltline may be reduced.
• the stirrer may be arranged to rotate about an axis passing through the lower part of the stirrer, or passing outside its lower part.
• the stirrer may be a rotary shaft and/or a rotor with at least one stirrer blade.
• Rotors may be of any type including prior art rotors as disclosed in the background description, such as high shear rotors provided with vanes for shearing gas and/or flux into fine bubbles, or pump action rotors in which gas and/or flux supply ducts are intercepted by mixing ducts for sucking in, mixing with the gas and/or flux, and expelling molten metal .
• the apparatus of the invention may be used in particular for the purification of molten aluminium, molten magnesium, cast iron, molten steel or copper, or for the separation of alloys thereof.
• the invention also relates to a rotatable stirrer of an apparatus for the purification of a molten metal as described above.
• the rotatable stirrer dips during operation in the molten metal, whereby upon rotation of the stirrer the molten metal is stirred and the fluid injected therein is dispersed.
• It comprises an upper part arranged to be engaged with a rotary drive means of the apparatus and a carbon-based lower part which during operation is partly immersed in molten metal up to a meltline.
• the carbon-based lower part has an interface portion extending from below to above the meltline.
• the interface portion is coated with a protective coating of a refractory material to improve the resistance against erosion, oxidation and/or corrosion during operation.
• a further aspect of the invention is a method of treating molten metal in an apparatus as disclosed above.
• the method comprises temporarily or permanently exposing the wear- exposed surface of the coated component to molten metal and imparting a rotary motion to the molten metal about a substantially vertical axis, the contacting molten metal being in motion relative to the wear-exposed surface.
• One particular embodiment of the method relates to the purification of molten metal in a purifying apparatus comprising or associated with a vessel for containing molten metal and a coated stirrer as described above.
• the method comprises injecting a purifying fluid, in particular a gas and/or a flux, into a molten metal contained in the vessel, and rotating the coated stirrer therein to stir and disperse the injected fluid in the molten metal to remove impurities towards the surface.
• the method of the invention may be applied to purify the molten metal either in batches or continuously.
• Different purifying gases may be injected into the molten metal to purify it, such as a halide gas, in particular chlorine and/or fluorine, or an inert or substantially inert gas, in particular nitrogen and/or argon.
• a halide gas in particular chlorine and/or fluorine
• an inert or substantially inert gas in particular nitrogen and/or argon.
• One gas or a mixture of different purifying gases may be injected into the molten metal .
• a flux for the purification process for instance a solvent such as an organic or inorganic fluoride, e.g. aluminium, magnesium or potassium fluoride powder or fused fluxes, carbonates, sulphates, nitrides and oxides, may also be injected into the molten metal as such or together with a purifying gas, the flux being usually in the form of a liquid, powder or granule.
• the purifying fluid is injected into the molten metal through a duct in the stirr
• the stirrer may be rotated at a speed of the order of 100 to 500 RPM, in particular in the range of 250 to 400 RPM.
• the coating is exposed to contacting molten metal that has a tangential speed of at least 0.1 m/s relative to the coated component.
• the relative speed may be in range of 1 to 10 m/s or even higher, in particular from 2.5 to 4 m/s.
• Another aspect of the invention is the use of a refractory material as protective coating against erosion, oxidation and/or corrosion on a rotatable stirrer of an apparatus for the purification of a molten metal.
• the invention also relates to the use of a carbon-based component coated with a protective coating of refractory material against erosion, oxidation and/or corrosion as rotatable stirrer in an apparatus for the purification of a molten metal.
• FIG. 1 schematically and by way of example shows part of one embodiment of an apparatus for the purification of a molten metal according to the invention, part of it being shown in an enlarged view and partly cut away;
• FIG. 2 schematically shows a stirrer dipping in molten metal and selectively coated with a refractory material according to the invention.
• the apparatus partly shown in Figure 1 comprises a vessel 20 containing molten metal 40, such as molten aluminium, to be purified.
• a rotatable stirrer 10 made of carbon-based material, such as graphite, is partly immersed in the molten metal 40 and is arranged to rotate and disperse a fluid injected therein.
• the stirrer 10 comprises a shaft 11 whose upper part is engaged with a rotary drive and support structure 30 which holds and rotates the stirrer 10.
• the lower part of shaft 11 is carbon-based and dips in the molten metal 40 contained in vessel 20.
• a rotor 13 At the lower end of the shaft 11 is a rotor 13 provided with flanges or other protuberances for stirring the molten metal 40.
• a gas supply (not shown) , for instance a gas reservoir provided with a gas gate leading to the flexible tube 35.
• the axial duct 12 is arranged to supply a fluid, e.g. gas and/or a molten or solid flux, to the rotor 13.
• the rotor 13 comprises a plurality of apertures connected to the internal duct 12 for injecting the fluid into the molten metal 40, as shown by arrows 51.
• the lower part of the shaft 11, i.e. the immersed part and the interface region at or about the meltline 14 of the shaft, as well as the rotor 13 are coated with a protective coating 18A of a refractory material which improves the resistance to erosion, oxidation and corrosion of the stirrer during operation.
• the upper part of the carbon- based shaft 11 is coated with a thin coating 18B of refractory material 18B providing protection against oxidation and corrosion, whereas the coating 18A protecting the immersed part of the shaft 11 and the rotor 13 is a thicker coating of refractory material providing protection against erosion, oxidation and corrosion.
• Such a coating gradation is adapted for coatings of stirrers used in batch processes, the lower part of the stirrers being exposed alternately to molten metal and oxidising atmosphere.
• the gradation may be inverted, i.e. thicker coating above the meltline and thinner coating below the meltline, or the coating may be equally thick above and below the meltline because the lower part of the stirrer is only exposed to molten metal, not to an oxidising atmosphere.
• the coatings 18A and 18B may be made of an aluminium- wettable material, such as a refractory hard metal (RHM) having little or no solubility in molten aluminium.
• RHM refractory hard metal
• Useful RHM include borides of titanium, zirconium, tantalum, chromium, nickel, cobalt, iron, niobium and/or vanadium, for example as disclosed in US patents 5,364,513 (Sekhar/de Nora) and 5,651,874 (de Nora/Sekhar) as well as in the examples below.
• a suitable coating consists of particulate refractory hard metal boride in a colloid applied from a slurry of the particulate refractory hard metal boride in a colloid carrier, wherein the colloid comprises at least one of colloidal alumina, silica, yttria, ceria, thoria, zirconia, magnesia, lithia, monoaluminium phosphate or cerium acetate, as described in US Patent 5,651,874 (de Nora/Sekhar).
• the colloidal carrier has been found to considerably improve the properties of the coating produced by non-reactive sintering.
• the method of application comprises applying to the surface of the component a slurry of particulate preformed refractory boride in a colloidal carrier as specified above, followed by drying, and by heat treatment.
• This method involves painting (by brush or roller) , dipping, spraying, or pouring the slurry onto stirrer 10 and allowing to dry before another layer is added.
• Each applied layer does not need to be entirely dry before the application of the next layer. It is preferred to heat the coating with a suitable heat source so as to completely dry it and improve densification of the coating.
• Heating and drying take place preferably in non-oxidising atmospheres at about 80-200°C, usually for half an hour to several hours, and further heat treatments are possible.
• stirrer 10 to be coated with this slurry may be treated by sand blasting or pickled with acids or fluxes. Similarly the surfaces may be cleaned with an organic solvent such as acetone to remove oily products and other debris prior to the application of the coating. These treatments will enhance the bonding of the coatings to the stirrer. After coating the surfaces of the stirrer by dipping, painting or spraying the slurry or combinations of such techniques in single or multi-layer coatings and drying, a final coat of the colloid alone may be applied lightly prior to use .
• the surfaces of the stirrer can be painted, sprayed, dipped or infiltrated with reagents and precursors, gels and/or colloids. To assist rapid wetting of the immersed part of stirrer
• the refractory coating may be exposed to molten aluminium in the presence of a molten flux assisting penetration of aluminium into the refractory material, the molten flux for example comprising a fluoride, a chloride or a borate, of at least one of lithium and sodium, or mixtures thereof.
• a molten flux assisting penetration of aluminium into the refractory material
• the molten flux for example comprising a fluoride, a chloride or a borate, of at least one of lithium and sodium, or mixtures thereof.
• the refractory material may be exposed to the corresponding metal before use, if appropriate in the presence of a flux.
• a reactive or non-reactive fluid in particular a gas alone or a flux, such as a halide, nitrogen and/or argon, is injected into the molten metal 50 contained in the vessel 20 through the flexible tube 35 and stirrer 10 which dips in the molten metal 40.
• the stirrer 10 is rotated at a speed of about 100 to 500 RPM so that the injected gas 50 is dispersed throughout the molten metal in finely divided gas bubbles.
• the dispersed gas bubbles 50 remove impurities present in the molten metal 40 towards its surface, from where the impurities may be separated thus purifying the molten metal.
• the stirrer 10 schematically shown in Figure 2 dips in a molten metal bath 40 and comprises a shaft 11 and a rotor 13.
• the stirrer 10 may be of any type, for example similar to the stirrer shown in Figure 1 or other designs known from the prior art.
• the rotor 13 of stirrer 10 may be a high-shear rotor or a pump action rotor.
• the interface portion at and about the meltline 14 of the carbon-based lower part of the shaft 11 is coated with a refractory interface coating l ⁇ A ⁇ for instance over a length of up to half that of the shaft 11.
• the length of coating 18A X could be a quarter of the length of shaft 11 or even less, depending on the design of stirrer 10 and the operating conditions.
• stirrer 10 in Figure 2 illustrates further coated surfaces which are particularly exposed to erosion.
• the lower end of the shaft 11 adjacent to the rotor 13 is protected with a coating 18A 2
• the lateral surface of rotor 13 is protected with a coating 18A 3
• the bottom surfaces of the rotor 13 is coated with a coating 18A 4 .
• the coating or different protective coatings on different parts of the stirrer such as coatings 18A 1# 18A 2 , 18A 3 and 18A 4 shown in
• Figure 2 may be adapted as a function of the expected lifetime of the stirrer.
• the amount and location of such coatings can be so balanced that they each have approximately the same lifetime.
• the coating on such stirrers may be continuous as illustrated in Figure 1 but with a graded thickness or composition so as to adapt the resistance against erosion to the intensity of wear of each part of the stirrer, thereby combining the advantages of the different coatings shown in Figure 2.
• the invention is understood not to be limited to the details of the apparatus shown in the Figures or described in the above specification. Additional embodiments and modifications within the scope of the claimed invention will be apparent to one of ordinary skill in the art.
• the shaft shown in Figure 1 may be modified so as to consist of an assembly whose non-immersed part is made of a material other than carbon-based, such as a metal and/or a ceramic, which is resistant to oxidation and corrosion and which, therefore, does not need any protective coating, whereas the immersed part of the shaft is made of carbon-based material protected with a protective coating according to the invention.
• a composite shaft would preferably be designed to permit disassembly of the immersed and non-immersed parts so the immersed part can be replaced when worn.
• a carbon-based non-immersed part of the shaft may be protected from oxidation and corrosion with a coating and/or impregnation of a phosphate of aluminium, in particular applied in the form of a compound selected from monoaluminium phosphate, aluminium phosphate, aluminium polyphosphate , aluminium metaphosphate, aluminium orthophosphate and mixtures thereof. It is also possible to protect the non-immersed part of the shaft with a coating and/or impregnation of a boron compound, such as a compound selected from boron oxide, boric acid and tetraboric acid.
• a boron compound such as a compound selected from boron oxide, boric acid and tetraboric acid.
• the protective coating may simply be applied to any part of the stirrer in contact with the molten metal, to be protected against erosion, oxidation and corrosion during operation.
• the stirrers shown in Figures 1 and 2 may each be surrounded by a tube-like stator extending from above into the molten metal 40 along shaft 11 down to above rotor 13 , the stator being arranged to contain between its internal walls and shaft 11 a purifying gas or flux for supplying the gas or flux to the molten metal 40 above rotor 13.
• the stator is preferably coated with a protective coating at least at and about a level corresponding to the meltline 14. Coatings of refractory material for protecting stirrers or other coated components according to the invention will be further described in the following examples .
• Example 1 Three non-treated conventional graphite shafts having a diameter of 60 mm were tested in a batch treatment process during which the shafts were rotated at a speed of about 200 rpm in molten Al-7Si-0.5Mg (A356), i.e. containing in weight% 92.5% Al, 7% Si and 0.5% Mg, at about 750-760°C. The tests were carried out in a standard Ramsell NaberTM electric resistance furnace .
• the shafts were extracted from the molten metal for examination and exposed to the oxidising atmosphere for approximately 20 min. After 15, 20 and 24 hours operation respectively, the three conventional shafts were coned down to such an extent that they needed to be replaced.
• Example 1 was repeated with three similar shafts but impregnated with aluminium orthophosphate by a conventional method in an autoclave and dried in an air oven.
• a slurry was prepared by mixing particulate TiB 2 , two grades of colloidal alumina and polyethylene glycol in the following proportions: lOOmg particulate TiB 2 -325 mesh (99% of particles smaller than 45 micron and a mean particle size of 6 micron) , 28 ml of a first grade of colloidal alumina (NYACOL®
• the surface of each shaft was mechanically roughened by shot blasting to increase anchorage of the coating.
• a coating was formed by brushing several layers of the above slurry onto the roughened surface of each shaft. Each applied layer was allowed to dry for a few minutes, typically 10 min., at 300°C before application of the next layer. After having reached a thickness of 800 micron, the slurry-applied coating was subjected to a heat treatment in an oxygen- containing atmosphere at 950°C for sintering and consolidation. Alternatively, the coating may be sintered and consolidated without prior drying heat treatment.
• the coated shafts were tested in a molten metal treatment process under the same conditions as in comparative Examples 1 and 2. After 168 hours operation, the shafts coated according to the invention were removed from the process . The graphite of each shaft and its coating had remained substantially intact and could have been further used.
• the shafts are coated with a slurry of 100 g particulate TiB 2 in 500 g of a cement consisting of approximately 57 weight% mullite (3A1 2 0 3 .2Si0 2 ) ,
• the waterglass is an aqueous solution containing 33.4 weight% sodium silicate in the approximate formula Na 2 0.3.8Si0 2 .
• each applied layer is preferably allowed to dry at room temperature or with heating up to about 200°C before application of the next layer until a desired thickness, e.g. 0.4 mm, is obtained.
• the coating is then heat treated at about 950°C for consolidation to form a glassy protective coating.
• the coated shaft can be used in molten aluminium as described in Examples 1 to 3 or for treating of other metals, such as magnesium, iron, steel or copper.
• Example 5 A further modification of Example 3 involves mixing
• the coating is consolidated under an inert atmosphere by heat treatment at a temperature of 1200°C followed by cooling whereby the aluminium-iron alloy particles are fused into a continuous matrix in which TiB 2 particles are embedded.
• Such a coated shaft can be used for treating molten metals up to a temperature of about 1100°C, e.g. for treating molten magnesium, aluminium or copper.

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• 2000
  • 2000-04-17 EP EP00920949A patent/EP1190203B1/de not_active Expired - Lifetime
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• 2002
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